Atlas V rockets launch from Pad 41. The closest possible launch viewing of any rocket launch from Cape Canaveral is offered for these sometimes, at just 2.3 miles away at the "LC-39 Observation Gantry," by buying tickets through the Kennedy Space Center Visitor Complex. Keep in mind this site could deemed off-limits at the last minute due to wind direction (a safety concern) because of the short distance to the pad, but this has only happened a couple of times so far.
Playalinda Beach is the next closest place, and also is the best spot outside the space center, at just 4.8 miles from the launch pad once you walk down the beach (5.7 from the first parking lot area). It is also the closest free or low cost (national park entrance fee) viewing site. However, it is not open for night launches (operating hours are 6 a.m. to 8 p.m. during daylight savings time summer months, and 6 a.m. to 6 p.m. during standard time winter months).
The Saturn V Center (5.4 miles), another stop on the KSC Visitor Complex tours, is an excellent option as well for Atlas V launches, accessed by taking the bus tour at the Visitor Complex with launch day tickets.
The Visitor Complex itself is 7.1 miles away, but offers no direct line of sight to the pad itself, which is behind the tree line. You'll see it once it lifts off.
For off-hours/night launches, when no tickets are being sold by the Visitor Complex and Playalinda Beach is closed, the best options for Atlas V launches are Port Canaveral on Route 528, or on the river in Titusville, both at 13 miles distance.